Many processes are carried out with the aid of compressors, these compressors frequently being of the type operating with a liquid as an auxiliary or working liquid. As the various gases are extracted or compressed, it is often the case that portions of these gases, e.g. owing to condensation, enter the auxiliary or working liquid of the compressor as undesirable impurities. Firstly, they may be environmentally harmful, i.e. they cannot simply be discharged with the working liquid of the pump. Moreover, they may also have physical and chemical properties which have such an adverse effect on the auxiliary or working liquid (vapor pressure/corrosion), that, when this liquid is fed back and reused in the compressor, reliable operation cannot be ensured. This may result, e.g., in the condensed proportions incorporated in the working liquid outgassing in the compressor, when the auxiliary or working liquid is fed back to the latter, and thus effecting a considerable reduction in the compressor capacity. It is known (EP-A-0 007 528) to clean such liquids by atomizing and heating at low pressure and by condensation. The effort involved is considerable and, in the case of large compressor processes, can undoubtedly be justified economically. In the case of relatively small systems, the effort, however, is disproportionately large. The object of the invention is to provide in this case an efficient, yet cost-effective option for cleaning the liquid.